Microtomes are used to cut thin sections from samples. These sections are subsequently placed on a coverslip, suitably processed, and then examined under a microscope. A microtome, in particular, a rotary microtome generally includes an object carriage. The object carriage carries a specimen holder that holds a specimen to be cut. The object carriage is moved up and down, by a drive means, in a vertical path on the rotary microtome. During this vertical movement, the specimen is passed over a cutter fixedly arranged on the rotary microtome.
In conventional rotary microtomes, control of the vertical cutting movement generally takes place by means of a transmission mechanism which is driven by a hand wheel. The transmission mechanism transforms turning movement of the hand wheel into vertical movement of the object carriage. In this type of drive, the moveable masses of the rotary microtome, which includes the specimen and all of the moveable components of the microtome, are alternately and repeatedly accelerated and decelerated. Gravitation accelerates the moveable masses during a first half-turn of the hand wheel (downward movement of the object carriage) and decelerates the moveable masses during the second half-turn of the hand wheel (upward movement of the object carriage). Thus, only a force reduced by the force of gravitation is required on the hand wheel during the downward movement of the object carriage, and a force increased by the force of gravitation is required during the upward movement.
To balance these undesired accelerations and decelerations, rotary microtomes include a mass-balancing arrangement. The mass-balancing arrangement generally comprises an asymmetrical balancing weight which is integrated into the hand wheel. However, where the moveable masses of the rotary microtome are relatively large, the balancing weight must be dimensioned to be correspondingly large, thereby increasing the bulk of the microtome. Furthermore, an asymmetrically formed balancing weight may produce undesirable vibrations in the rotary microtome during relatively fast upward and downward movements of the object carriage. Vibrations in the microtome inevitably cause the specimens to be cut in such a way that they are unusable.
U.S. Pat. No. 5,881,626 discloses a mass-balancer for balancing the movable masses of the microtome. The mass-balancer has a pre-tensioned, adjustable spring element and a pivotably mounted lever for compensating different inertial forces of the microtome in combination with the spring element. A drawing element connects the lever to the object carriage. The mass-balancer is indirectly connected to the drive mechanism by the drawing element. In particular, the lever has, extending from a pivot pin, an upper lever arm and a lower lever arm, which are arranged at an angle with respect to each other. The force acting on the object carriage can be adapted appropriately in all positions by the leverages provided by the lever arms and the tension spring acting on the lower lever arm.
However, in U.S. Pat. No. 5,881,626, when the object carriage moves downwardly, the spring is further tensioned, the effective length (moment arm) of the lower lever arm is shortened gradually, while the spring force, which changes with the moving distance and angles, is increased in accordance with a trigonometric function. Thus, it is impossible that the product of the moment arm and the spring force remains constant. On the other hand, the weight or acting force of the object carriage mass remains constant, but the moment arm of the object carriage mass is changed, so the product of the moment arm and the acting force of the object carriage mass cannot be constant. Under this circumstance, it is impossible to reach a perfect balance between the two sides of the lever.